Phosphate-bonded grinding wheel

ABSTRACT

An abrasive body of grit-size particles of alumina or silicon carbide held together by a water-insoluble aluminum phosphate bonding matrix, which body is of increased strength through its phosphate bonding matrix containing a small amount of at least one or both of particlate aluminum and an in-situ formed phosphosilicate; and an improved molding composition for preparing the abrasive body, which molding composition contains up to 12 percent by weight thereof of particulate aluminum of minus 200 mesh size and/or sodium silicate having a SiO2 to Na2O weight ratio greater than 1 and less than 3.8.

United States Patent [1 1 Sheets, Jr. et al.

[ Nov. 6, 1973 PHOSPHATE-BONDED GRINDING WHEEL [75] Inventors: Herbert 1). Sheets, Jr., Columbus,

Ohio; Ronald A. Meyer, Beloit, Wis.; Martin J. OHara, Columbus, Ohio [52] 1.1.5. C1..'. 51/308, 51/309 [51] int. Cl 824d 3/02, C09c 1/68 [58] lFieid of Search 51/295, 298, 307,

[56] References Cited UNITED STATES PATENTS 3/1959 Eubank ..5 /30s 2 1951 Masin an ..5l/308 3,071,456 1/1963 Cheesman 51/307 Primary ExaminerDonald J. Arnold Att0rney.l0seph R. Spalla [57] ABSTRACT An abrasive body of grit-size particles of alumina or sil icon carbide held together by a water-insoluble aluminum phosphate bonding matrix, which body is of increased strength through its phosphate bonding matrix containing a small amount of at least one or both of particlate aluminum and an in-situ formed phosphosilicate; and an improved molding composition for preparing the abrasive body, which molding composition contains up to 12 percent by weight thereof of particulate aluminum of minus 200 mesh size and/or sodium silicate having a SiO to Na O weight ratio greater than 1 and less than 3.8.

12 Claims, No Drawings PHOSPHATE-BONDED GRINDING WHEEL This invention relates to strength improvement of an abrasive body held together by a water-insoluble phosphate binder. More particularly the invention concerns (a) an improved molding composition containing a small amount of at least one strength-promoting agent selected from the group consisting of particulate aluminum of fine size and sodium silicate of moderate to high SiO to Na O weight ratio, and (b) an improved abrasive body of superior strength prepared from the molding composition.

In a commonly owned copending allowed application entitled Phosphate Bonded Gridning Wheel, Serial No. 768,184, filed Oct. 16, 1969, now US. Pat. No. 3,619,151, issued November 9, 1971, there are disclosed a phosphate-bonded abrasive body, and a molding composition for its preparation, of the particular nature and type for which the present invention provides significant improvements thereover by employing therein certain specific strength-promoting agents. This disclosure incorporates from some to all portions of the afore-identified application as are of interest and con- (:em to supplement, complete, and further disclose the present invention. Thus, in conjunction with teachings herein and those of theafore-identified application, it should be apparent that abrasive bodies comprise abrasive grit particles held together by a binder or matrix of another material with these abrasive bodies in the form of grinding wheels, coated surfaces, abrasive tips and/or tool inserts being of utility for removing material, deburring, grinding, smoothing, polishing, and like operations for preparing useful articles.

As taught in the afore-identified application, its phosphate-bonded abrasive bodies are of the following formulation:

ABRASIVE BODY COMPOSITION* 1 Generally Useful" Preferred Abrasive Grit 3060 45-56 Phosphate Bonding Matrix At least 8 and 20-28 up to balance Voids Up to 45 Balance Other (e.g., Fillers, etc.) O-45 -10 To Total w 100 100 Of the afore-identified US. Ser. No. 768,l84 Percent by volume The afore-identified application also teaches its mixed cohesive molding compositions most generally useful and particularly preferred for preparing its useful'abrasive bodies are of the following formulations:

MOLDING COMPOSITION FORM ULATIONS* Preferred The afore-identified application further teaches: its molding compositions contain an abrasive selected from the group of abrasive materials consisting of synthetic arid natural aluminum oxides (alumina) and silicon carbide; the abrasive is employed in the form of grit particles of that size grade or gradations requisite for the particularly intended application of the abrasive body being prepared therefrom; and specific and particularly useful abrasive particle grit sizes "for employment in preparing abrasive bodies which are a grinding wheel and lapping and polishing wheels. Additionally taught in the afore-identified application as essential constituents, for providing the phosphate bonding matrix for the abrasive grit particles in the abrasive body, are phosphoric acid, fine alumina particles, and finely divided ferric oxide Fe,0, and useful amounts of those constituents. The afore-identified application teaches those essential constituents as follows;

The term phosphoric acid as used herein is intended to encompass and include (a) orthophosphoric acid, (b) mixtures containing at least 40 percent by weight of orthophosphoric acid-with pyrophosphoric acid, tripolyphosphoric acid, and other like higher polymeric phosphoric acids, and (0) aqueous solutions of (a) and (b) of limitedfree water content. The useful aqueoussolutions, employed in the molding composition, shduld be of a low free water content and a high enough phosphoric acid concentration that in their employed amount the free water therein constitutes less than about 3 percent by weight of the molding composition formulation.

The fine alumina particles employed in the formulation generally are of a particle size of less than 325 mesh (U.S. Sieve Series). The employed ferric oxide is finely divided and most desirably of a particle size of less than about 3 microns. When employing the minimum amount, an extremely small portion thereof, up to about 5 percent of the minimum amount can be of larger particle sizes, and when employing amounts greater than the minimum amount larger porportions can be of particles greater than 3 microns.

Additionally the afore-identified application teaches its useful molding composition on an optional basis may include up to several percent, to about 5 percent, by weight of an inorganic filler material, such as expanded perlite (sodium potassium aluminum silicate), asbestos fibers, and the like as wellas'up to severalpercent by weight ofceramic bond strengthening agents, wetting and/or dispersing agents, and other materials as will be obvious to those in the art."

It now has been discovered that the phosphatebonded'abrasive body taught in the afore-identified application is' significantly improved and is provided with asignificantly higher and superior strength through including in the molding composition from which it is prepared an effective yet small amount of at least one strength-promoting agent selected from the group consisting of a fine-size particulate aluminum and sodium silicate of moderate to high SiO to Na O weight ratio. Although the afore-identified application teaches that its molding composition on an optional basis may include up to several percent by weight of ceramic bond strengthening agents, it is believed to be hitherto unknown and unobvious that fine particulate aluminum and certain sodium silicates each intentionally included separately and together in small amount (a) function as significant bond-strengthening agents in molding compositions for preparing aluminum phosphate-bonded abrasive bodies, and (b) provide abrasive bodies whose aluminum phosphate bonding matrix contains a small content of particulate aluminum and/or in-situ formed phospho-silicate, included separately or together as the case may be depending on the particularly employed molding composition, and whose strength is vastly superior to like prepared abrasive bodies lacking that small content.

The present invention includes a mixed cohesive molding composition for cold-pressing and subsequent heat-curing to become an article containing abrasive grit particles held together by a water-insoluble phosphate binder, which molding composition is of the following most generally useful and preferred formulations:

IMPROVED MOLDING COMPOSITION FORMULATION Generally Preferred Useful Abrasive Grit (Silicon 90-30 62-78 Carbide or Alumina) Phosphoric Acid 3-21 11-6 325 Mesh Alumina Particles 646 25-15 Finely Divided Ferric Oxide 0.4 up to about 2-1 of 3 one-third of micron size the 325 mesh particles alumina particles Strength-Promoting Agent effective amount (Particulate aluminum to 12 4 to 2% of 200 mesh size, or

sodium silicate having a SiO, to Na,0 weight ratio greater than 1 and less than 3.8, or

both) Other (e.g., Fillers) 10 0-5 To Total 100 100 Parts by weight The present invention also includes an abrasive body prepared from the foregoing improved molding compositions, which abrasive body in general is non-distinguishable exteriorly by eye from the abrasive body of the afore-identified application. It comprises the same abrasive body composition of the afore-identified application except (a) it includes in its aluminum phosphate matrix a minor amount of up to percent by volume of at least one or of both of particulate aluminum of less than 200 mesh size and in in-situ formed phospho-silicate, and (b) it possesses a significantly greater strength than the abrasive body of the aforeidentified application.

By the present invention providing an abrasive body of significantly improved strength there accrues the following important advantages:

a. the wheel is capable of being used at higher speed and still can withstand centrifugal forces without spalling;

b. advantageously smaller amounts and proportions of this inventions phosphate bonding matrix can provide abrasive bodies having strengths heretofore only obtainable with larger amounts and proportions of the heretofore phosphate bonding matrix. They The particulate aluminum useful as a strengthpromoting agent in the invention are particles of aluminum metal. Useful aluminum metal particles may be rough or smooth surfaced and regular or irregular in shape and size, as desired. They may be prepared by any known procedure, such as comminution of massive size aluminum metal to the requisite particle size. The particulate aluminum employed should be of a fine size and of a size less than 200 mesh Larger particles of particulate aluminum are not useful in that they react more slowly with the phosphoric acid. Most generally one utilizes particulate aluminum metal of between -200 mesh and +325 mesh size.

The sodium silicate useful as a strength-promoting agent in the invention are the sodium salts or esters of silicic acid. They commonly are known as water glass or soluble glass. The invention utilizes sodium silicates of moderate to high SiO to Na O weight ratio. The more strongly alkaline silicates are known to be more easily water-solubilized and are detrimental to obtaining an abrasive article non-degradable by water and the like Coolants, should a residue of the more strongly alkaline silicate remain in unreacted form in the aluminum phosphate matrix of the article. Most generally the employed sodium silicate has a SiO' to Na O weight ratio greater than 1.0 and less than 3.8, and most preferably of a weight ratio between 2.0 and 3.3. The sodium silicate is utilized in its hydrated form and most generally of a concentration between 2.0 and 3.0 Baume 20 for mixing with the abrasive grit.

The strength-promoting agent is utilized in an effective amount to provide the abrasive article with significantly increased strength. Some increased strength is.

noted in the article upon utilizing an effective amount as low as 0.1 part by weight thereof in each parts by weight of the molding composition from which the article is prepared. Most generally, however, and to provide a significant strength improvement, one includes between 0.3 and 12 parts of the strengthpromoting agent per 100 parts by weight of the molding composition, and preferably includes between 2.5 and 4 parts by weight. It will be noted from the specific examples, presented later, that in general the article s strength increases as the amount utilized of the agent is increased. Amounts of the strength-promoting agent above about 12 parts in 100 parts of the molding composition generally are no more effective, and may lesson grinding efficiency. The strength-promoting agent can be either the particulate aluminum, or the sodium silicate, or both may be included.

As will be apparent from what follows, the mixed cohesive compositions of the invention are prepared by a procedure closely alike that taught in the aforeidentified application for preparing its molding composition except for modification of that procedure to provide for incorporating the strength-promoting agent in the compositions formulation. In general, the molding compositions of the aforeidentified application are prepared: by muller-mixing or the like mixing of the finely divided ferric oxide and the fine alumina particles together to provide a thoroughly mixed blend of mixed oxide fine particles; the abrasive grit particles and phosphoric acid also are mixed and blended together until the surfaces of the grit particles are wetted by the phosphoric acid, such as by mixing together for several minutes by a rapidly moving, bladed-cage mixer or the like; and then the two prepared blends are mixed thoroughly together.

To prepare the molding composition of the invention, the foregoing procedure is modified as follows: when the employed strength-promoting agent is sodium silicate, a hydrated form of the sodium silicate is mixed and blended with the abrasive grit and then, in most instances, this blend dried or otherwise treated to remove a large portion of the water from the employed sodium 5 silicate solution prior to mixing the abrasive grit/- sodium silicate blend with the phosphoric acid and proceeding with balance of the foregoing preparation procedure. The sodium silicate is utilized in a hydrated form so as to facilitate a thorough wetting of the abra- 5 sive grit particles. Although not always necessary (depending on type and amount of sodium silicate), water is removed from the applied sodium silicate solution, after it has been applied to the abrasive grit, to avoid exothermic heating upon a blending of the phosphoric acid. This water removal need not be a complete removal of all water. Generally it is accomplished by drying at moderate temperatures and by any of numerous techniques for water removal such as are illustrated in the specific examples. When the employed strength- 15 prepared blends have been mixed thoroughly together, one adds and blends therewith the particulate aluminum. When the strength-promoting agent to be included in the molding composition is to be both the particulate aluminum and the sodium silicate, then a combination of the above procedures are followed with the sodium silicate being blended with the abrasive grit prior to blending therewith the phosphoric acid and with the particulate aluminum being blended into the mixed oxide blend or blended into the molding composition last, as desired.

The invention now is described further by presenting specific illustrative embodiments, or examples, of this inventions molding composition containing the strength-promoting agent, and for comparison purposes of like molding compositions lacking the strength-promoting agent. Each of these molding composition formulations is prepared by those procedures just described. In the following Table 1, the numberidentified formulations are comparison formulations according to the teachings of the aforeidentified application, and the letter-identified formulations are examples of the improved molding composition formulation of the invention.

TABLE 1.MOLD1NG COMPOSITION FORMULATIONS" Formulation identification 1 A B C Abrasive/phosphoric acid blend:

No. 60-grit size silicon carbide No. 60-grit size alumina......... No. 320-grit size silicon carbide No. 320-grit size ulumina.....

Phospholeum" 6.65 6.62 6.74 6.67 5.76 5.71 5.80 5.75 9.05 8.83 7.07 6:56 6:68 Mixed oxide blend:

325 mesh alumina particles 25.29 25.12 22.36 22.20 21.82 21.70 19.28 19.15 18.60 18.16 15.89 16.22 13.37 Finely divided ferric oxide (less than 2 micron size) 1.36 1.35 1.22 1.20 1.18 1.17 1.04 1.03 1.00 0.98 0.86 0.86 0.72 Strength-promoting agent:

Particulate aluminum 2.26 1.95 Sodium silicate 0.67 0.58 2.45

Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

TABLE 1.Continued Formulation identification 5 .1 K L M N o 6 P Q R s T Abrasive/phosphoric acid blend:

No. (SO-grit size silicon carbide 68.48 68.48 68.48 68.48 68.48 68.48 68.48 62.14 62.14 62.14 62.14 62.14 62.14

No. 60-grit size alumina No. 320-grit size silicon carbide No. 320-grit size alumina.. Phosphoelum Mixed oxide blend:

325 mesh alumina particles 21.66 21.66 21.66 21.66 21.66 21.66 21.66 28.33 28.33 28.33 28.33 28.33 28.33 Finely divided ferric oxide (less than 2 micron size) 1.14 1.14 1.14 1.14 1.14 1.14 1.14 1.49 1.49 1.49 1.49 1.49 1.49 Strength-promoting agent:

Particulate aluminum" 2.20 2.20 2.20 2.20 2.20 2.20 Sodium silicate 0.34 0.69 1.03 1.37 1.71 2.06 "1.54 1.41 1.24 1.45 1.43

Total 100.00 100.34 100.69 ,l01.03 101.37 101.71 102.06 100.00 101.54 101.41 101.24 101.45 101.43

TABLE l. Continued Formulation identification 7 U V W X Y Z AA BB (.(f DD Abrasive/phosphoric acid blend:

No. (ill-grit size silicon carbide 62.79 62.79 62.79 No. 60- zrit size alumina No. 32(1-grit size silicon carbide. Nu. 320-grit size alumina.... Phospholeum Mixed oxide blend:

- 325 mesh alumina particles... Finely divided ferric oxide (less Strength'promoting agent:

Particulate aluminum" Sodium silicate Total 100.00 101.88 101.88 101.88 101.88 100.37 100.73 101.09 101.46 101.82 I( Parts by weight Registered trademark of Monsanto Chemical Company for a 105 percent phosphoric acid consisting essentially of an equilibrium mixture of about 58 percent by weight oforthophosphoric acid, 38 percent by weight of pyrophosphoric acid. 3.5 percent by weight of tripolyphosphoric acid, and a trace of higher polymer acids.

ZOO-mesh aluminum powder. such as No. 123 aluminum powder of Alcoa Corporation.

" Aqueous sodium silicate employed: I Percent $10 to N3 Examples 50/1115 Be uf. ratio S mh as A through F..l through 0. and Y through DD... 36-38 40-42 2.40 40-42 Be of the Sargent Company. (I through 1. Q. and U through X... 42.9 47.0 2.90 PQ silicate K of the Philadelphia Quartz Company. P 32.10 52.5 2.38 No. 16 silicate (technical grade) of E. l. du Pom de Nemours and Company. 32.05 35 3,75 PQ silicate -35 of the Philadelphia Quartz Company. 37.60 41 3.22 PQ silicate N of the Philadelphia Quartz Company. 44.10 50.5 2.00 PO silicate D of the Philadelphia Quartz Company.

in preparing the improved molding composition forof the mold, contacting the molding compositions, can mulations tabulated in Table l, which contain the sobe coated with a silicone mold release agent. The emdium silicate as the strength-promoting agent, care is ployed cold-pressing means is a hydraulic press for apused during application of the sodium silicate solution plying pressure to compact the molding composition toto the abrasive grit and the drying thereof to remove a predetermined stop providing a cold-pressed mass of water to avoid agglomerating of grit particles into clusthe desired thickness. Following the cold pressing, the ters. Generally and for most formulations the hydrated mass is procured and final cured. in most instances a sodium silicate is added slowly to an agitated mass of standardized precure of 1 /2 to 2 hours at 170F to the grit particles and agitation continued during a water 190F is employed. The precure is accomplished by removal by heating for several hours between 50 and placing the cold-pressed mass in a preheated oven 260C. As illustrative of various useful mixing and dryequipped with electrical resistance heating elements. ing techniques: in Formulation Example U, the hy- The employed oven is cold at the initiation of the predrated sodium silicate is added while mixing the silicon curing of some compositions. in others the oven is at carbide grit by means of a high sp d lad d miXcr precure temperature when the cold-pressed mass is means and then the sodium silicate-wetted grit is dried la ed in the oven for procure. by tumbling while heated under infraed lamps 110 Following precuring of the cold-pressed masses, each about Formulation the Same Procedure is is finally cured. In most instances the heat in the oven f llowed except th W tt d and par i lly dried g U- is increased so that the precured mass heats at the rate sodium silicate blend is dried still more by heating addif b t 100F r hour to 650F; thi 650F temperati n lly at 500F for 2 hours; in Formulation Ex mpl ture is held for about four hours; and then the cured W, the hydrated sodium silicate lS t0 and mixed removed from the oven en oyed after allowi the Silicon Carbide g under agitation, While a ing the oven to cool to a lower temperature as desired). flame is directed onto the surface of the agitated grit; I th i ces, the r cur d mass has its temperaa d i1! Formulation pl the g is Pfewetted ture raised gradually, or in step-wise increments, at a adiluted hydrated sodium silicate, tumble dried rate no greatet than 600F per hour and usually at a forcing heated air therethrough, and then ball milled rate f about 100 h to a temperature f at four mihutas to break P agglomerates and clusters least 600F and, is held above this temperature for at of the Sodium Silicate-coated E least 2 hours. Higher final curing temperatures also are Each of the molding compositioh's formulations in useful, but are not necessary to provide formation of a Preceding Table I is Processed y cold-Pressing, P water-insoluble phosphate binder and an improved curlhg, and final curing to Provide a useful abrasiveabrasive body of increased strength. Temperatures held together y a water'lnsoluble Phosphate above about 1,200F generally are avoided as abrasive binder general this further Processing includes P bodies cured above that temperature are of no greater mg a Precalculated Weighed amour" of molding strength than are obtainable when the final curing is ac- Posilion 11110 a Suitable mold Cavity and Spreading it complished within the range of 600F to 1200F. therein to a layer of relatively uniform thickness The The following Table II tabulates strength data values p y Weighed amount of molding composition is for abrasive bodies prepared from each of the molding that pp y an amount of abrasive grit particles 1 composition formulations presented in Table 1. In some calculated on a volume basis to provide the particular instances the bend strength f the abrasive body was abrasive grit loading desired in the cold-pressed interim determined and in o'ther instances there was deterabrasive body- Generally tool Steel cylinders of pp mined the abrasive body's diametrical tensile strength. Prime inside diameter are p y for the mold in From the strength data presented in Table II, it will be which to prepare wheel-like abrasive bodies, such as apparent that every l d i d l i 3-inch thick grinding Wheels having an OD of 18 position formulation (e.e., those containing particuinches and an ID. of 14 inches. If desired, the surfaces l d l i d/ h di ili t rovided an 9 abrasive body of significantly higher strength than the strength exhibited by the abrasive body prepared from the comparison formulation lacking the strengthpromoting agent and otherwise most closely approximating the same molding composition formulation.

TABLE ll. ABRASIVE BODY Diametral tensile From molding Abrasive Bend strength (psi) strength (psil composition grit formulation (%/vol.) Range Av. Range Av.

(6) 1160-1280 1230 P.. 1650-1730 1700 Q. 1570-1680 1610 R. 1300-1600 1490 S.. 1290-1580 1460 T.. 1540-1720 1600 (7) 1060-1640 1270 U. 1470-1670 1580 V. 1530-1670 1600 W. 1330-1480 1420 X. 1400-1720 1570 Y 2405-2705 2602 Determined Bend Strength= w 4 2WT- thickness.

"Determined in substantial accordance with the teachings in Materials Resmrz'h and Standards. April, 1963. pages 283-88.

W= width; T=

Each of the abrasive bodies tabulated in Table 11 pos- I sesses adequate resistance to dissolution to water and like coolant solutions to be useful for known abrading purposes where such coolant solutions are used in contact with the abrasive body. As illustrative thereof, an abrasive body of Example Q, after soaking in water for ten days and then drying, has substantially the same tensile strength of about 1,600 psi as a like abrasive body not so treated, and also another abrasive body of Example Q, after water soaking and while thoroughly wetted, retained about 80 percent of its tensile strength in comparison to a like abrasive body not so treated.

While the invention has been described in detail and illustrated by specific examples herein, it will be apparent to those skilled in' the art that some changes and modifications can be made thereof without departing from the true spirit and scope of the invention which is encompassed by the claims which follow.

We claim: 7

1. In a mixed cohesive molding composition for coldpressing and subsequent heat-curing to become an article containing abrasive grit particles held together by a water-insoluble phosphate binder, comprising between an effective amount and up to 12 parts by weight of at least one strength-promoting agent selected from the group of particulate aluminum of less than 200 mesh size and sodium silicate, having a SiO to Na O 10 weight ratio greater than 1 and less than 3.8, in each 100 parts by weight of said composition, from to 30 parts by weight of abrasive grit particles selected from grit-size particles of alumina and silicon carbide, from 3 to 21 parts by weight of phosphoric acid, from 6 to 46 parts by weight of minus 325 mesh alumina particles, and finely divided ferric oxide in an amount of at least 0.4 parts by weight and up to about one-third the parts of the minus 325 mesh alumina particles.

2. The composition of claim 1 including the strengthpromoting agent which is said particulate aluminum.

3. The composition of claim 1 including the strengthpromoting agent which is said sodium silicate.

4. The composition of claim 1 including both said particulate aluminum and said sodium silicate as the strength-promoting agent.

5. An improved mixed cohesive molding composition for cold-pressing and subsequent heat-curing to become an article containing abrasive grit particles held together by a water-insoluble phosphate matrix, which composition for each parts by weight thereof comprises:

a. between 4 and 2.5 parts by weight of at least one strength-promoting agent selected from the group consisting of particulate aluminumof less than 200 mesh size and sodium silicate having a SiO to Na O weight ratio between 2.0 and 3.3;

b. from 62 to 78 parts by weight of abrasive-grit particles selected from the group consisting oflalumina and silicon carbide;

c. from 11 to 6 parts by weight of phosphoric acid whose free water content constitutes less than 3 percent by weight of the composition;

(1. from 25 to 15 parts by weight of 325 mesh alumina particles; and

e. from 2 to 1 parts by weight of less than 3 micron size ferric oxide particles.-

6. The composition of claim 5 including the strength-.. promoting agent which is said particulate aluminum.

7,. The composition of claim 5 including the strengthpromoting agent which is said sodium silicate.

8. The composition of claims including both said particulate aluminum and said sodium silicate as the strength-promoting agent.

9. An abrasive body comprised of:

a. abrasive grit particles selected from the group consisting of grit-size particles of alumina and silicon carbide in an amount constituting from 30 to 60 percent by volume of said body; held together by b. a water-insoluble phosphate bonding matrix constituting at least 8 percent by volume of said body, which water-insoluble bonding matrix consists essentially of aluminum phosphates and a minor amount of iron phosphates intermingled therewith and includes a minor amount up to 10 percent by volume of at least one or of both of particulate altiminum of less than 200 mesh size and an in-situ formed phospho-silicate.

10. The body of claim 9 which contains the particulate aluminum.

11. The body of claim 9 which contains the phosphosilicate.

12. The body of claim 9 which contains both the particulate aluminum and the phospho-silicate. 

2. The composition of claim 1 including the strength-promoting agent which is said particulate aluminum.
 3. The composition of claim 1 including the strength-promoting agent which is said sodium silicate.
 4. The composition of claim 1 including both said particulate aluminum and said sodium silicate as the strength-promoting agent.
 5. An improved mixed cohesive molding composition for cold-pressing and subsequent heat-curing to become an article containing abrasive grit particles held together by a water-insoluble phosphate matrix, which composition for each 100 parts by weight thereof comprises: a. between 4 and 2.5 parts by weight of at least one strength-promoting agent selected from the group consisting of particulate aluminum of less than 200 mesh size and sodium silicate having a SiO2 to Na2O weight ratio between 2.0 and 3.3; b. from 62 to 78 parts by weight of abrasive-grit particles selected from the group consisting of alumina and silicon carbide; c. from 11 to 6 parts by weight of phosphoric acid whose free water content constitutes less than 3 percent by weight of the composition; d. from 25 to 15 parts by weight of -325 mesh alumina particles; and e. from 2 to 1 parts by weight of less than 3 micron size ferric oxide particles.
 6. The composition of claim 5 including the strength-promoting agent which is said particulate aluminum.
 7. The composition of claim 5 including the strength-promoting agent which is said sodium silicate.
 8. The composition of claim 5 including both said particulate aluminum and said sodium silicate as the strength-promoting agent.
 9. An abrasive body comprised of: a. abrasive grit particles selected from the group consisting of grit-size particles of alumina and silicon carbide in an amount constituting from 30 to 60 percent by volume of said body; held together by b. a water-insoluble phosphate bonding matrix constituting at least 8 percent by volume of said body, which water-insoluble bonding matrix consists essentially of aluminum phosphates and a minor amount of iron phosphates intermingled therewith and includes a minor amount up to 10 percent by volume of at least one or of both of particulate aluminum of less than 200 mesh size and an in-situ formed phospho-silicate.
 10. The body of claim 9 which contains the particulate aluminum.
 11. The body of claim 9 which contains the phospho-silicate.
 12. The body of claim 9 which contains both the particulate aluminum and the phospho-silicate. 